Sparks from battery when hooking it up - might be normal?

[jtexas]

Putting the cranking batt in the boat Saturday morning, hooked up the negative leads, touched the positive to the post and *POP* big old spark. I'm talking about the main leads from the starter motor and the engine ground. <br /><br />I visually inspected the cables, couldn't find any problems. Then (luckily before I opened up the throttle box) realized I had the black cable connected to the positive post. Hooked it up correctly, and everything worked normally after that.<br /><br />With the negative cable connected to the "+" post, measured 12.0 volts from the positive cable to the "-" battery post, about a .65v drop vs. voltage across the battery.<br /><br />I know it sounds like this should be posted in "Stupid Human Tricks," but I'm very curious about whether this is normal and why it does that.<br /><br />Any opinions, insights, comments?<br /><br />thanks,<br />jtw

 

[jlinder]

Re: Sparks from battery when hooking it up - might be normal?

If you have it correctly connected and have a voltage drop across a cable, you have a current flow.<br /><br />If everything is turned off, you have a problem somewhere. My guess is you already know this by the dead battery you have.<br /><br />If I am correctly reading your post, the only thing to do is to track down where the draw is. Try removing fuses one at a time until the voltage drop goes away.

 

[Ralph 123]

Re: Sparks from battery when hooking it up - might be normal?

Let me tell you what this sounds like, assuming I've understood your post correctly - <br /><br />Most systems have some sort of reverse polarity protection to protect the system from being fried if the power is hooked up backwards. The quickest and cheapest way is a diode of some sort. <br /><br />When power is hooked up correctly the diode (which sits across the +/- of the system) looks like an open - it does not conduct and is essentially invisible to the system.<br /><br />When the power is hooked up backwards, the diode conducts and basically looks like a short so that the system beyond the diode is protected. A lot of current will flow because when a diode is conducting it drops only around 0.65V<br /><br />So it sounds like the protection diode did it's job. That is why you saw the spark. I bet the cables got warm too.<br /><br />My only concern would be how long you had it like that and if too long, that diode got damaged. That all depends on the rating (size really) of the diode.<br /><br />If the diode should fail as an open (won't conduct either way) the worse that could happen is you've lost your future RP protection (i.e., next time it happens stuffs going to fry).<br /><br />If it fails as a short your battery will short no matter how the cables are hooked up.<br /><br />Now, I'm not sure where that diode is in the system. You'd need a schematic and possibly the schematics of the equipment in the system. The diode could have been internal to a component. In which case it is possible only that component was protected.<br /><br />I hope this makes sense. If not, ask away and I'll try to explain myself better.

 

[jtexas]

Re: Sparks from battery when hooking it up - might be normal?

Jack, thanks for the feedback; my description might have been a little obtuse. It sparked when I reversed polarity, but functioned normally when correctly connected. The battery seems unaffected.<br /><br />Ralph, sounds like you hit it right on the head. It was only connected in reverse long enough for one big spark; I didn't try again. Got out the voltmeter so I could determine which lead it was since I have one for the starter, one for the tilt, and a couple more for accessories.<br /><br />That must be one big-@$$ diode. I don't think I smoked it; the battery didn't drain while I was fishing. Charging system appears to have functioned normally. I measured the voltage while it was still reversed, but after it sparked, then measured 0 volts (red cable to "+" post with the black cable connected to "-" post). Everything worked during the day.<br /><br />Hope you're right, I'll try tracking it down & testing it. It's a '79 Rude 70hp, not too complicated. I have the Seloc manual which has some schematics.<br /><br />thanks very much,<br />jtw

 

[Ralph 123]

Re: Sparks from battery when hooking it up - might be normal?

You should be fine then. They can usually take large currents for short periods w/o a problem. I was worried you measured the drop while it was hooked up.<br /><br />FYI - this kind of RP protection is often called crowbarring - as in putting a crowbar (dead short) across the power source (battery) when polarity is reversed.

 

[jlinder]

Re: Sparks from battery when hooking it up - might be normal?

The crowbar approach is basic. Draw high current and blow the fuse. The crowbar (diode?) usually survives since the fuse goes quick.<br /><br />If your battery does not disharge just sitting there it sounds like you are fine.

 

[demsvmejm]

Re: Sparks from battery when hooking it up - might be normal?

Originally posted by Ralph:<br /> You should be fine then. They can usually take large currents for short periods w/o a problem. I was worried you measured the drop while it was hooked up.<br /><br />FYI - this kind of RP protection is often called crowbarring - as in putting a crowbar (dead short) across the power source (battery) when polarity is reversed.

Ralph, finally someone who is really knowledgeable and can explain. Thank you for your thorough and concise, yet not condescending replies. I am an ASE certified auto technician and knwo about the diode in automotive circuits, but never would have thought about it in a marine application. <br />And it's good to know as I have done this myself and I'm sure I'll have fun this spring reinstalling my battery as I have at least three wires for each battery connection. Hope I zip-tied them when I took the battery out

 

[18rabbit]

Re: Sparks from battery when hooking it up - might be normal?

JTW - Whenever you connect a battery, always connect the [+] post first, and then connect the [-] post last. When disconnecting, reverse the order, remove the [-] first and [+] last.

 

[Ralph 123]

Re: Sparks from battery when hooking it up - might be normal?

Thanks for the compliment David. I really appreciate that. I have learned much here and I just try to give back whenever I can.